It has been customary practice to measure the rate of flow of a fluid by heating the fluid with thermal pulses at an upstream position in a pipe through which the fluid flows, detecting the heated fluid at a position spaced a certain distance downstream from the position in which the fluid has been heated, and measuring an interval of time which has elapsed after the fluid has been heated and before the heated fluid is detected, for thereby measuring the speed of flow of the fluid. The average speed of flow v can be expressed as follows: EQU v=L/S (cm/sec) (1)
where L is the rate of flow of the fluid (ml/sec) and S is the cross-sectional area of the pipe through which the fluid flows.
Assuming that a heater unit located upstream in the pipe for heating the fluid is spaced from a thermosensitive unit disposed downstream of the heater unit in the pipe by a distance d (cm), and an interval of time T elapses after the fluid has been heated by the heater unit and before the heated fluid is detected by the thermosensitive unit, the following equation can be derived: EQU T=1/L S.multidot.d (sec) (2)
As can be seen from the equation (2), as the rate of flow L of the fluid increases, the time interval T decreases in inverse proportion. Conventional hot-wire flowmeters have heated the fluid periodically. It has been difficult to set the period at which the fluid is to be heated by the heater unit in a system in which the rate of flow of the fluid is subjected to large changes. More specifically, if the heating period were too large, then the measurement time would be prolonged and rapid changes in the rate of flow of the fluid could not be followed, resulting in a failure in proper measurements in which such rapid changes are reflected. If the heating period were too short, then the fluid as detected downstream in the pipe per unit time would be adversely affected by heating in a next period, with the result that detection of the heated fluid would be inaccurate.
Fluids to be measured are of a variety of compositions, available in a wide range of rates of flow, and are measured for their rates of flow under various conditions such as ambient temperature, pressure, and the like. Under such various conditions, the fluids are heated by the heater unit under varied conditions, and cannot be heated to a constant temperature. For example, as the speed of flow of the fluid increases, an increased amount of thermal energy is lost from the heater which is then reduced in resistance, and hence the fluid is heated less intensively. If the heated portion of the fluid had different temperatures at the thermosensitive unit, it would be unable to detect the heated portion of the fluid reliably.
It is an object of the present invention to provide a thermal pulse flowmeter having a wide range of measurements.
Another object of the present invention is to provide a thermal pulse flowmeter capable of measuring a fluid flowing at rapidly and largely changing rates, such as an expiration or inspiration.